allow dyn_sym in the files where they are defined; remove unreachable android code

Author: RalfJung

src/tools/miri/src/machine.rs

@@ -707,7 +707,8 @@ impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {
                 );
             }
             "android" => {
-                // "signal"
+                // "signal" -- just needs a non-zero pointer value (function does not even get called),
+                // but we arrange for this to be callable anyway (it will then do nothing).
                 let layout = this.machine.layouts.const_raw_ptr;
                 let ptr = this.fn_ptr(FnVal::Other(DynSym::from_str("signal")));
                 let val = ImmTy::from_scalar(Scalar::from_pointer(ptr, this), layout);

src/tools/miri/src/shims/unix/android/foreign_items.rs

@@ -6,11 +6,12 @@ use shims::foreign_items::EmulateForeignItemResult;
 
 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriInterpCx<'mir, 'tcx> {}
 
-pub fn is_dyn_sym(name: &str) -> bool {
-    matches!(name, "signal")
+pub fn is_dyn_sym(_name: &str) -> bool {
+    false
 }
 
 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
+    #[allow(unused, clippy::match_single_binding)] // there isn't anything here yet
     fn emulate_foreign_item_inner(
         &mut self,
         link_name: Symbol,
@@ -21,15 +22,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         let this = self.eval_context_mut();
 
         match link_name.as_str() {
-            "signal" if this.frame_in_std() => {
-                let [_sig, _func] =
-                    this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
-                this.write_null(dest)?;
-            }
             _ => return Ok(EmulateForeignItemResult::NotSupported),
         }
 
-        #[allow(unreachable_code)]
         Ok(EmulateForeignItemResult::NeedsJumping)
     }
 }

src/tools/miri/src/shims/unix/foreign_items.rs

@@ -20,6 +20,23 @@ use shims::unix::freebsd::foreign_items as freebsd;
 use shims::unix::linux::foreign_items as linux;
 use shims::unix::macos::foreign_items as macos;
 
+fn is_dyn_sym(name: &str, target_os: &str) -> bool {
+    match name {
+        // `signal` is set up as a weak symbol in `init_extern_statics` so we might as well allow it
+        // in `dlsym` as well.
+        "signal" => true,
+        // Give specific OSes a chance to allow their symbols.
+        _ =>
+            match target_os {
+                "android" => android::is_dyn_sym(name),
+                "freebsd" => freebsd::is_dyn_sym(name),
+                "linux" => linux::is_dyn_sym(name),
+                "macos" => macos::is_dyn_sym(name),
+                target_os => panic!("unsupported Unix OS {target_os}"),
+            },
+    }
+}
+
 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriInterpCx<'mir, 'tcx> {}
 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
     fn emulate_foreign_item_inner(
@@ -237,14 +254,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 this.read_target_usize(handle)?;
                 let symbol = this.read_pointer(symbol)?;
                 let name = this.read_c_str(symbol)?;
-                let is_dyn_sym = |name| match &*this.tcx.sess.target.os {
-                    "android" => android::is_dyn_sym(name),
-                    "freebsd" => freebsd::is_dyn_sym(name),
-                    "linux" => linux::is_dyn_sym(name),
-                    "macos" => macos::is_dyn_sym(name),
-                    target_os => panic!("unsupported Unix OS {target_os}"),
-                };
-                if let Ok(name) = str::from_utf8(name) && is_dyn_sym(name) {
+                if let Ok(name) = str::from_utf8(name) && is_dyn_sym(name, &this.tcx.sess.target.os) {
                     let ptr = this.fn_ptr(FnVal::Other(DynSym::from_str(name)));
                     this.write_pointer(ptr, dest)?;
                 } else {

src/tools/miri/src/shims/unix/linux/foreign_items.rs

@@ -12,8 +12,8 @@ use shims::unix::linux::sync::futex;
 use shims::unix::sync::EvalContextExt as _;
 use shims::unix::thread::EvalContextExt as _;
 
-pub fn is_dyn_sym(_name: &str) -> bool {
-    false
+pub fn is_dyn_sym(name: &str) -> bool {
+    matches!(name, "getrandom")
 }
 
 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriInterpCx<'mir, 'tcx> {}